Okay, I'm gonna give you a very short simple answer. Metals are elements that have the ability to lose electrons and on the other hand, non-metals are the elements that tend to gain electrons. Both metals and non-metals do this to attain noble gas configuration (i.e. fill their valence shell completely). Noble gases are already stable, hence they do not behave as either acids nor bases.

I think you mean the Group VIII A elements. This group is now called Group 18.

According to the Brønsted–Lowry acid-base theory, acids are able to donate protons(or `H^+` ions) and bases are able to accept protons(or `` ions). Now the elements of Group 18 have complete duplets(Helium) and octets(Neon, Argon, Krypton, Xenon, radioactive Radon). That is why they have little tendency to participate in chemical reactions as the sole purpose of chemical reactions itself it attainment of stability, which these elements already have. Thus they generally do not donate electrons or form compounds. So when they are introduced in water, they can neither accept nor donate protons as they are already stable. Hence Group 18 elements are neither acidic nor basic.

Just for your extra knowledge, scientists have managed to create compounds of noble gases in the laboratory. Interestingly, with reference to your question, Xenic Acid can also be created, which is a compound containing Xenon, a Group 18 element. But naturally, these elements are non-reactive and hence neither acidic nor basic. Hope this helps!

Acids, according to the Brønsted–Lowry theory, are proton donors. An example is hydrochloric acid [HCl(aq)]. Gaseous HCldissociates in water to produce hydrogen ions (H+) which are protons. When reacting with NaOH (aq), HCl(aq) donates H+ ions to the solution, causing H+ and OH- to form water. And it is the protons that give hydrochloric acid its acidic nature.

Bases, according to the Brønsted–Lowry theory, are proton acceptors. An example is sodium hydroxide (NaOH). When reacting with HCl(aq), it accepts a proton (H+) from HCl. Thus, the hydroxide ions (OH-) present in NaOH (aq) form water by accepting the H+ ions from HCl(aq).

To summarise the above two paragraphs, we have this ionic equation:

H+ (aq) + OH- (aq) --> H2O (l)

Here, H+ is the proton. Any substance that can donate a proton is a Brønsted–Lowry acid. And OH- is the base (alkali as it is aqueous) because it accepts the hydrogen ions from the acid.

And now it is the question. Why are noble gases not acidic or basic? The reason lies in whether they can donate or accept protons.

Helium, for instance, exists as simple discrete atoms moving around the nucleus at great speeds. In other words, it is a monoatomic gas. It has 2 electrons in its valence shell; we say the shell is fully occupied. Helium is chemically unreactive and stable because it has a stable noble gas configuration. Same for Neon and Argon that all their valence shells are fully occupied, having a stable electronic configuration.

Since noble gases are highly unreactive, they do not react or form bonds with other substances. So there is no chance that they would donate or accept protons. As a result, they are neutral.

I think you mean group 18 aka the noble gases. The reason they are not acidic or basic is because they already have all their valence electrons filled, therefore they are already stable and neutrals. Other elements on the periodic table lose/gain electrons to gain their a pseudo noble gas configuration which makes them basic or acidic.